The outdoor units and indoor units of air conditioning systems are generally provided with heat exchangers for exchanging heat with the outside air. The heat exchangers are normally provided with a plurality of heat radiating fins, a plurality of heat exchanger tubes, and a propeller or other blowing means. The plurality of heat radiating fins are plate-like members arranged with a prescribed spacing there-between along the thickness direction thereof. The plurality of heat exchanger tubes are arranged so as to pass through the plurality of heat radiating fins in the thickness direction of the heat radiating fins. The blowing means serves to deliver a flow of air over the plurality of heat radiating fins and heat exchanger tubes.
In such a heat exchanger, the air blown into the gaps between adjacent heat radiating fins by the blowing means causes heat to be exchanged and the refrigerant flowing inside the heat exchanger tubes either evaporates or condenses.
The heat radiating fins generally comprise a plate material that has been worked into a prescribed fin shape using a die. The plate material is constructed from a plate-like substrate made of pure aluminum or an aluminum alloy and a coating that is formed on the surface of the substrate. Normally, the substrate is made by rolling the aluminum or aluminum alloy into a plate-like form using rolling oil and, thus, oil exists on the surface of the substrate.
Consequently, conventionally, when a coating is to be formed on the surface of the substrate, an oil removal treatment is first applied to the substrate and then a coating method like one of the following (for example) is used to form the coating:
(1) A chromic acid treatment is applied to form a chromate coating and a hydrophilic coating is formed over the chromate coating. (See, for example, Japanese Publication No. 62-105629.)
(2) A hydrophilic paint-like material is applied to form the coating. (See, for example, Japanese Publication No. 63-303729.)
(3) A phosphoric acid treatment, an alkaline treatment, etching, or other surface roughening treatment is applied to the substrate and then a corrosion resistant coating is formed on the substrate. (See, for example, Japanese Publication No. 2003-171774.)
With the method (3), a fine roughness (protrusions and depressions) of submicron order is formed on the surface of the substrate.
With the method (1), the equipment cost is large because it is necessary to use a prescribed treatment layer and the running cost is high because harmful waste liquid is discharged from the process and must be treated. Thus, it seems reasonable to omit the chromic acid treatment and select a method such as method (2) in which a paint-like material is applied directly to the substrate using a paint application method. However, when the paint-like material is applied directly, the adhesion between the substrate and the coating is poor and a sufficiently durable plate material cannot be obtained. Meanwhile, when the method (3) is adopted, specific treatment equipment is required and the cost is high.
Also, all of the methods (1) to (3) require the substrate to be treated in advance to remove oil and when a roughening treatment is applied, the number of processing steps required to manufacture the plate material increases and causes the cost to increase.
In the case of a conventional plate material made by applying a chromic acid treatment to the substrate so as to form a chromate coating on the surface of the substrate, the chromate coating improves the corrosion resistance of the plate material. However, since a chromate coating does not have sufficient hydrophilicity, it has poor water wetting characteristic when used as is on a heat radiating fin. As a result, water droplets stuck on the surface of the fin cause such problems as increased air flow resistance.
In view of these issues, technologies have been proposed in which a chromate coating or water soluble resin coating is applied to the surface of the substrate as a corrosion resistant coating and a hydrophilic coating is applied on top of the corrosion resistant coating in order to ensure sufficient degrees of both corrosion resistance and hydrophilicity. (See, for example, Japanese Publication No. 62-105629.) With the technology described in Japanese Publication No. 62-105629, a plate material having sufficient degrees of both corrosion resistance and hydrophilicity can be obtained. Particularly when a water soluble resin is used for the corrosion resistant coating, the treatment layer required for a chromic acid treatment is not necessary and the techonology is thus advantageous in terms of production costs and environmental concerns.